High purity antenna windows used in applications such as tactical missiles, re-entry vehicles, decoys and the like, are currently made from boron nitride. Antenna windows made from boron nitride are disadvantageous due to non-congruent vaporization at the temperatures to which the antenna windows are subjected during use. The inclusion of impurities such as silica, in the boron nitride affects the melting point, and blackout problems occur due to the non-congruent vaporization at temperatures at which the antenna windows are subjected during use.
Since aluminum nitride is known to vaporize congruently, blackout problems in antenna windows should not be encountered at elevated temperatures as in the case of antenna windows made with boron nitride.
Aluminum nitride is a refractory material, and it does not sublime appreciably at normal pressures until the temperature is in the range of about 2000.degree. C. to about 2450.degree. C. In U.S. Pat. No. 4,172,754, aluminum nitride is described as having useful electrical properties, notably as a thin film insulator and as a piezoelectric material of high surface-acoustic-wave velocity. Several prior art processes for making aluminum nitride are described in U.S. Pat. No. 4,172,754, which is incorporated herein by reference. In one prior art process ammonia and aluminum trichloride are combined in the gaseous phase in a multi-stage reaction to produce solid aluminum nitride and gaseous hydrogen chloride. U.S. Pat. No. 4,172,754 also describes the prior art process of making aluminum nitride by the organometallic process wherein ammonia is reacted in the gaseous phase in stages with aluminumtrimethyl to yield aluminum nitride solid and gaseous methane. This process is described as being disadvantageous because simple organometallic compounds tend to be unstable, and the final reaction stage tends to be homogeneous resulting in the deposition of aluminum nitride powder on the growing layer of aluminum nitride.
In other prior art processes, such as that described in U.S. Pat. No. 2,962,359, aluminum nitride is obtained by heating at a high temperature alumina and carbon in a nitrogen stream.
In U.S. Pat. No. 3,397,958, aluminum nitride is made relatively free of iron, silicon and titanium by the treatment of a bauxite (an impure aluminous material containing silica, iron oxide and titanium oxide) with carbon and sulfur at elevated temperatures in the presence of nitrogen. Other prior art processes of preparing aluminum nitride are described in U.S. Pat. No. 3,147,076 and U.S. Pat. No. 3,032,398.
The aluminum nitride made by the prior art processes generally contains sufficient impurities so that the aluminum nitride cannot be used to make high purity antenna windows. In most of the prior art processes, the entrainment or inclusion of impurities result in sufficient contamination of the antenna windows so that they cannot be used in the applications discussed above.
Although the prior art describes the use of aluminum-containing organometallic compounds for the formation of thin film passivating layers in integrated circuitry, such deposits are typically thin and non-crystalline. In antenna windows, it is necessary that the windows be made from crystalline deposits of high purity aluminum nitride.